Rap1, a Ras-like GTPase with a distinct function

Rap1 is a very close relative of Ras, but its function is likely to be distinct. A novel assay was developed to measure activation of Rap1 and it was found that a large variety of extracellular stimuli are able to activate this small GTPase Very rapidly. Common second messengers, like calcium, diacylglycerol and cAMP mediate this activation in a cell type dependent manner. We have studied the molecular mechanism of these activations and one of the striking results was the discovery of Epac, a novel binding protein of cAMP. Epac is a guanine nucleotide exchange protein for Rap1. This finding indicates that the common notion that all effects of cAMP are mediated by protein kinase A is incorrect The function of Rap1 is largely unknown. Recent evidence indicates that the hypothesis that Rap1 functions as an antagonist of Ras effector signalling is probably incorrect. More like Rap1 mediates a distinct pathway regulating processes related to the control of the actin cytoskeleton affecting vesicular traficking and/or cell adhesion.